Add a lint for `expect` when the expected and actual arguments don't have compatible types

[bwilkerson]

It would be nice to have a lint to catch uses of the expect from package:test where the expected and actual arguments do not have compatible types. Right now both parameters are typed as dynamic so the type system doesn't really help. But it's uncommon for the objects to be equal if the types are not at least assignable. For example,

expect(3, ''); // LINT

When omitting types from local variables it's easy for changes elsewhere to cause the variable's type to change, which can lead to test failures. It would be nice to find these during analysis rather than during test execution.

[srawlins]

CC @natebosch

[natebosch]

We could probably cover some common cases, but i don't think we can solve the general case.

Anyone can define a Matcher and there is no way to carry expected input types with a Matcher definition, so we'd need some kind of annotation to describe allowed types.

I think @davidmorgan started exploring this.

[bwilkerson]

True, but if the expected is anything other than a Matcher then I think we could flag inconsistent types. I don't know about other code bases, but a fair portion of the analyzer code base uses actual values for expected rather than matchers (mostly integers and strings).

But you might be right, it might not be worthwhile for other teams.

[davidmorgan]

Yes, see cl/362510313.

Seems likely there is a worthwhile lint here..